Document FROM <image>:<tag> Dockerfile instruction.

Upstream-commit: 1a8a540209c997f5e75a476b942365aad892ad35
Component: engine

components/engine/docs/sources/use/builder.rst

@@ -68,6 +68,10 @@ building images.
 
     ``FROM <image>``
 
+Or
+
+    ``FROM <image>:<tag>``
+
 The ``FROM`` instruction sets the :ref:`base_image_def` for subsequent
 instructions. As such, a valid Dockerfile must have ``FROM`` as its
 first instruction. The image can be any valid image -- it is
@@ -81,6 +85,9 @@ especially easy to start by **pulling an image** from the
 to create multiple images. Simply make a note of the last image id
 output by the commit before each new ``FROM`` command.
 
+If no ``tag`` is given to the ``FROM`` instruction, ``latest`` is
+assumed. If the used tag does not exist, an error will be returned.
+
 3.2 MAINTAINER
 --------------